let arr = [1,2,[3,[4]]]
function flattenArray(arr){
    let newArr =[]
    for(let i = 0;i <arr.length;i++){
        if(Array.isArray(arr[i])){
            newArr=newArr.concat(flattenArray(arr[i])) 

        }else{
            newArr.push(arr[i])
        }
    }
    return newArr
}
console.log( flattenArray(arr));
